This invention relates generally to devices for sensing the level of a liquid in a container and, more particularly, to devices of this kind that utilize a tuning fork or the like whose resonant frequency changes when contacted by the liquid.
Devices of this particular kind generally include a tuning fork disposed at a fixed location within the container, with both an excitation transducer and a receiving transducer mechanically coupled to the tuning fork. An oscillator generates an electrical excitation signal that is coupled to the excitation transducer, to mechanically oscillate the tuning fork. The receiving transducer senses this oscillation and produces a receive signal having an amplitude and frequency corresponding to the sensed oscillation. The tuning fork's oscillation is damped by the medium (i.e., gas or liquid) in which it is disposed, and its resonant frequency generally varies according to whether it is disposed in the gas or the liquid.
Typically, the excitation signal produced by the oscillator has a frequency that is made to sweep a frequency band that encompasses the tuning fork's resonant frequency both when disposed in the gas and when disposed in the liquid. By determining the particular frequency within this range at which the amplitude of the receive signal is a maximum, the tuning fork's resonant frequency can be determined. Alternatively, a series of pulses having a known frequency spectrum can be applied to the excitation transducer, and the resulting receive signal can be analyzed in a frequency discriminator to determine the tuning fork's resonant frequency. From the resonant frequency determination, the presence of the gas or liquid adjacent to the tuning fork can be determined.
In other level sensing devices of this kind, the resonant frequency of the tuning fork can be determined using a phase discriminator that monitors the relative phases of the excitation signal applied to the excitation transducer and the receive signal produced by the receiving transducer. Such devices utilize a recognition that the phase of the receive signal changes significantly at or near the tuning fork's resonant frequency.
Although the level sensing devices described briefly above can function reliably to detect the presence or non-presence of a liquid at a particular location within a container, the devices are believed to be unduly complicated and to consume an excessive amount of electrical power. There is therefore a need for an improved level sensing device that can reliably sense the presence or non-presence of a liquid at a particular location within a container, with reduced complexity and with reduced power consumption. The present invention fulfills this need.